According to the developer's website the Bluetooth Special Interest Group (SIG) did not define in the Bluetooth specification a transport layer specific for PC Card or PCI devices as they did for USB. The Apple Bluetooth software supports USB devices following the specification but can't do for PC Cards. Many Bluetooth PC Card manufactures have adopted proprietary protocols. One famous transport layer is the Cambridge Silicon Radio (CSR) BlueCore Serial Protocol (BCSP). Other manufactures have created their own transport layer which is normally not available. Some cards use a transport layer called UART described in Bluetooth Core specification part H4. This driver tries to implement H4 for the 3Com Bluetooth PC Card (Version 3.0).